The present invention relates to a magnetic medium (CoCrTa) comprising a nonmagnetic substrate such as glass or an Al-Mg alloy, a thin Cr underlayer RF sputtered on the substrate, and a CoCrTa magnetic layer RF sputtered on the Cr underlayer, and the preparation method of the magnetic medium.
It is very common that magnetic recording media such as disks, as well as magnetic tapes are used in recording and storing computer data, video and audio signals, etc. The higher the recording density of the magnetic materials, the lesser the area required for recording. In other words, these media will fulfill the increasing trends of light weight, thinness, and small size required for electronic products applications. However, high coercivity (Hc) is needed in order to obtain high recording density for the magnetic medium. Therefore, much effort has been put into research in order to obtain thin magnetic recording media of excellent recording density.
Owing to the fact that the residual magnetization and coercivity of thin films are higher than that of oxide powders, the conventional hard disks coated with oxide powders have been changed to thin film disk. An appropriate magnetic material is sputtered onto a nonmagnetic metallic underlayer to form a thin metallic medium of layered structure to increase the Hc and consequently recording density of the magnetic medium.
FIG. 1. illustrates the layered structure of a sputtered thin film hard disk. Aluminium substrate (1) is first electrolessly plated with nickel phosphate (2) and then placed into a sputtering compartment. The Cr underlayer (3) Co based alloy magnetic film (4) and carbon protective layer (5) are sequentially sputtered with thicknesses of 2500 A, 600 A and 300 A. When the sputtering procedure is completed, the disk is subjected to a lubricating treatment (6) and a testing procedure.
Cr underlayer is used to improve the coercivity of the magnetic medium by epitaxial growth of the magnetic grains on the well textured Cr grains. And, the recording density of the medium is thus increased.
Magnetic layer materials such as CoNiCr (Co-30% Ni-7.5% Cr or Co-20% Ni-10% Cr) or CoCrTa alloys, etc., are substantially used for data storage. The carbon film is used to protect the metallic layers underneath from the corrosive environment and to prevent them from being worn by the movement of the magnetic head.
In order to increase the coercivity of the recording medium so as to increase the recording density, not only should the recording properties of the magnetic medium be improved, but the Cr underlayer should also be studied. This is due to the fact that the Cr underlayer influences the coercivity of the medium dramatically. In particular, the thickness of the Cr underlayer has a great influence on the coercivity of the medium. Research has demonstrated that the coercivity of the medium increases as the thickness of Cr underlayer increases. When the thickness of the Cr underlayer is 2500 A, the coercivity thereof is approaching the saturation value. However, to sputter Cr underlayer to such a thickness, much time and materials are needed. Thus, the cost of production is high and unfavorable for mass production. Furthermore, crystal defects are likely to be formed if the thickness of the Cr film is over 2500 A. The defect may be one of the factors which causes noise in recording and reproducing.
Prior art shows that for CoNiCr medium to possess a coercivity of greater than 1000 Oe, the thickness of the Cr underlayer should be greater than 2000 A. For the CoCrTa/Cr medium, to obtain a coercivity greater than 1100 Oe, the thickness of the Cr underlayer should also be greater than 2500 A.
IEEE Transacions on Magnetics, Mag-22, No. 5, Sept. 1986, pp573-575 disclosed an alloy sputtered disk (developed by the Institute for Super Materials, ULVAC Corp.) The alloys sputtered are Co-30Ni-7.5Cr and Co-20Ni-10Cr film series. The underlayer of the disk is Cr. If the thickness of the underlayer is 200 A, the coercivity of the media is only 500 Oe. Only when the thickness of the Cr underlayer is larger than 2000 A, will the coercivity be 1000 Oe.
In IEEE Transaction on Magnetics, MAG-22, No. 5, p334, 1986, G. L. Chen disclosed a CoNiCr/Cr medium deposited by DC Magnetron Sputtering. It is found that epitaxial crystal growth is formed. This is due to the fact that the atomic stacking density of Co HCP(10T1) and that of Cr bcc(110) are approximately equal.
The epitaxial growth makes the C axis parallel to surface of the CoNiCr film, thus enhancing Hc. However, a magnetic layer with coercivity of 1000 Oe needs a Cr underlayer at least 2500 A thickness.
R. D Fisher, in IEEE Transaction on Magnetics, MAG-23, No. 5, p352, September, 1986 disclosed the new Co-14Cr-2Ta film for longitudinal recording medium. Under its sputtering conditions, the Cr underlayer is 2500 A, the coercivity is approaching the saturation value of 1100 Oe.
J. Appl. Phys. 63(8), p3260, April 1988 disclosed a CoCrM/Cr medium, wherein M is a third element and if the thickness of the Cr underlayer is over 2500 A, the coercivity of the medium is approaching saturation. As a result of this, the S/N ratio is approximately 4 dB higher than that of the conventional disk.
Japanese Patent JP 1133217, published in May, 1989, issued to Victor Co. disclosed a CoCrTa/Cr medium for longitudinal recording. The medium is obtained by DC Magnetron sputtering under an argon pressure of 5 mtorr. The composition in at % for a 600 A thick magnetic layer is Co-12%Cr-6%Ta for a 1000 A thick Cr underlayer, the maximum coercivity is 1200 Oe.
As indicated above, the thickness of the Cr underlayer of CoNiCr/Cr for use in longitudinal recording should be about 2500 A or above so that the coercivity of the medium can be approaching saturation. For a CoCrTa/Cr medium, in order for the coercivity to reach 1200 Oe, the required thickness of the Cr